Combined Effects of Foundry Sand Replacement and Polypropylene Fiber Content on Slag Geopolymer Mortar Properties

Ground granulated blast-furnace slag (GGBFS)-based geopolymer mortar is a low-carbon alternative to ordinary Portland cement. However, its reliance on the availability of natural river sand and its inherent brittleness are major constraints from the viewpoint of environmental sustainability and resistance to cracking. Because the combined effect of waste foundry sand (WFS) and polypropylene (PP) fibers in slag geopolymers is largely unexplored, the present research examined the possibility of using WFS to partially replace river sand and PP fibers for reinforcement of GGBFS-based geopolymer mortar. Twelve mixtures were prepared in which river sand was replaced by WFS at 0%, 25%, and 50% by volume, and 0%, 1%, 2%, and 3% PP fiber (by mass of binder) was incorporated into the mixtures. The effects of WFS and PP fiber on workability and hardened properties were analyzed using compressive strength, flexural strength, ultrasonic pulse velocity, water absorption, electrical resistivity, and modulus of elasticity. An increase in WFS content led to a decrease in workability and hardened properties. The greatest reduction in compressive strength was observed at 50% WFS, with a 46.0% reduction after 28 days (43.6% at 25% WFS). The addition of PP fiber also led to reductions in flowability, compressive strength, ultrasonic pulse velocity, and electrical resistivity. However, an improvement in flexural strength was observed with PP fiber addition across all WFS replacement levels. The maximum increase in flexural strength was 21.6% with 2% PP fiber, while at 3% fiber, flexural strength decreased slightly but remained higher than that of the fiber-free samples. Fractured specimens exhibited increased cohesion after cracking and reduced crack widths with increasing PP fiber content. Overall, the combined use of WFS and PP fibers can improve waste utilization and crack resistance, although the associated reduction in compressive strength should be considered in mix selection.